Ammonia Generation and Utilization in a Passive SCR (TWC+SCR) System on Lean Gasoline Engine 2016-01-0934

Lean gasoline engines offer greater fuel economy than the common stoichiometric gasoline engine, but the current three way catalyst (TWC) on stoichiometric engines is unable to control nitrogen oxide (NO_X) emissions in oxidizing exhaust. For these lean gasoline engines, lean NO_X emission control is required to meet existing Tier 2 and upcoming Tier 3 emission regulations set by the U.S. Environmental Protection Agency (EPA). While urea-based selective catalytic reduction (SCR) has proven effective in controlling NO_X from diesel engines, the urea storage and delivery components can add significant size and cost. As such, onboard NH₃ production via a passive SCR approach is of interest. In a passive SCR system, NH₃ is generated over a close-coupled TWC during periodic slightly rich engine operation and subsequently stored on an underfloor SCR catalyst. Upon switching to lean operation, NO_X passes through the TWC and is reduced by the stored NH₃ on the SCR catalyst. In this work, a passive SCR system was evaluated on a 2.0-liter BMW lean burn gasoline direct injection engine to assess NH₃ generation over a Pd-only TWC and utilization over a Cu-based SCR catalyst. System NO_X reduction efficiency and fuel efficiency improvement compared to stoichiometric engine operation were measured. A feedback control strategy based on cumulative NH₃ produced by the TWC during rich operation and NO_X emissions during lean operation was implemented on the engine to control lean/rich cycle timing. At an SCR average inlet temperature of 350 °C, an NH₃:NO_X ratio of 1.15:1 (achieved through longer rich cycle timing) resulted in 99.7 % NO_X conversion. Increasing NH₃ generation further resulted in even higher NO_X conversion; however, tailpipe NH₃ emissions resulted. At higher underfloor temperatures, NH₃ oxidation over the SCR limited NH₃ availability for NO_X reduction. At the engine conditions studied, greater than 99 % NO_X conversion was achieved with passive SCR while delivering fuel efficiency benefits ranging between 6-11 % compared with stoichiometric operation.